1. Field of the Invention
The invention relates to an integrated circuit with an output stage that includes an analog amplifier connected, on the output side, to a terminal pad for the outputting of an amplified output signal.
In integrated circuits with analog signal processing, operational amplifiers or operational transconductance amplifiers (OTA), for example, are provided for outputting output signals to lines external to the circuit. As a result, the output signal is output with high driver power from the integrated circuit to a line on a circuit board. In conventional operational amplifiers or OTAs as presented for example in Tietze, Schenk: xe2x80x9cHalbleiterschaltungstechnikxe2x80x9d, [xe2x80x9cSemiconductor Circuitryxe2x80x9d], 9th Edition, 1991, page 140; Hogervorst, Huijsing: xe2x80x9cDesign of Low-Voltage, Low-Power Operational Amplifier Cellsxe2x80x9d, pages 150 and 188; Laker, Sansen: xe2x80x9cDesign of Analog Integrated Circuits and Systemsxe2x80x9d, page 487, a capacitive feedback path is connected to the output terminal. The amplifiers are connected up with feedback networks, each individual amplifier stage effecting a phase shift in the signal path. To avoid positive feedback signal components, high frequency components are attenuated by the capacitive feedback mentioned.
A bonding wire is usually connected to the terminal pad for the outputting of the output signal from the integrated circuit, the bonding wire connecting the terminal pad to a terminal pin of the housing. This configuration acts as a receiving antenna for high-frequency interference radiations that can be generated by adjacent circuit components, for example. The signal path that is formed by the capacitive feedback but has low resistance for high frequencies leads from the terminal pad back into the circuit. The high-frequency interference radiation, so-called electromagnetic compatibility (EMC) interference, which is substantially received by the bonding wire is rectified by the pn junctions present in the integrated circuit. The currents injected as a result shift the operating points of the circuits in the integrated circuit. Inputs of amplifier stages are usually embodied with high resistance. As a result, currents so injected noticeably interfere with the signal processing.
The reference De Bruyn, William: xe2x80x9cError feedback in audio powerxe2x80x9d, Electronics World, June 1997, pages 476 to 478, in particular, FIG. 1 therein, shows an analog amplifier with a first amplifier stage, containing a differential amplifier, and also an output stage, in the case of which the output is fed back through resistive and capacitive elements to the output of the differential amplifier stage. The reference Tietze, Schenk: xe2x80x9cHalbleiter-Schaltungstechnikxe2x80x9d shows differential amplifiers with a first amplifier stage and a second amplifier stage connected downstream, in the case of which the output of the amplifier stage is fed back to the input, in particular, in the 9th Edition, 1990, pages 140 and 433, and the 11th Edition, 1999, page 523. The principle of amplifiers with nested Miller compensation is described in Pernici, Sergio et al.: xe2x80x9cA CMOS Low-Distortion Fully Differential Power Amplifier with Double Nested Miller Compensationxe2x80x9d, IEEE Journal of Solid-State Circuits, Volume 28, No. 7, July 1993, pages 758 to 763.
It is accordingly an object of the invention to provide an integrated circuit with an analog amplifier that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that has an output-side analog amplifier with a higher strength with respect to electromagnetic coupling-in.
With the foregoing and other objects in view, there is provided, in accordance with the invention, an integrated circuit, including an input terminal for feeding in an input signal to be amplified, an output terminal for outputting an amplified output signal, a terminal pad connected to the output terminal for outputting the output signal, a terminal for a reference-ground potential, and an analog amplifier having a first input-side amplifier section having a first signal path and an output, a second output-side amplifier section having a second signal path, the first and second signal paths connected in series and a transistor having a control terminal coupled to the output of the first input-side amplifier section and a controlled current path coupled to the terminal for the reference-ground potential and to the output terminal, a series circuit having a resistor, a capacitive element, a node between the resistor and the capacitive element, and ends, one of the ends of the series circuit connected to the output terminal, and another of the ends of the series circuit connected to the control terminal, and a further capacitive element connected between the node and the terminal for the reference-ground potential.
According to the invention, the integrated circuit has an analog amplifier, which includes an input terminal for feeding in an input signal to be amplified and also an output terminal for outputting an amplified output signal, a terminal pad, which is connected to the output terminal of the analog amplifier and serves for the outputting of an output signal from the integrated circuit, the analog amplifier including a first input-side amplifier section and a second output-side amplifier section, whose signal paths are connected in series, and a series circuit including a resistor and a capacitive element, which is connected at one end to the output terminal of the analog amplifier, a node being formed between the resistor and the capacitive element of the series circuit, in which case the second amplifier section includes a transistor, whose control terminal is coupled to an output of the first amplifier section and whose controlled current path is coupled to a terminal for a reference-ground potential and to the output terminal of the analog amplifier, in which case the series circuit including the resistor and the capacitive element is connected at the other end to the control terminal of the transistor, and in which case a further capacitive element is provided, which is connected between the node and the terminal for the reference-ground potential.
In the case of the integrated circuit according to the invention, interference coupled in at high frequency through the output is attenuated. The capacitive element connected relative to reference-ground potential acts together with the resistor as a low-pass filter so that the coupled-in signal energy of the high-frequency interference is delayed and discharged to reference-ground potential. As a result, less charge is injected into the interior of the integrated circuit. Although low-frequency interference is, nevertheless, conducted further, it is detected by the control dynamic range of the analog amplifier and corrected.
The analog amplifier may be, for example, an operational amplifier that has a high-resistance input, effects analog amplification of the useful signal, and provides a low-resistance signal on the output side. By contrast, an OTA has a high-resistance output that supplies a current-controlled output signal. Operational amplifiers and OTAs are of multistage construction. A differential amplifier stage that operates differentially is provided on the input side, a further amplifier being connected downstream of the stage on the output side. The further amplifier substantially includes an amplifier transistor. The output of the analog amplifier and the input of this second amplifier stage exhibit capacitive feedback, which feedback includes the series circuit formed by a resistor and a capacitive element, the coupling node between capacitive element and resistor, in turn, being capacitively connected to reference-ground potential. The resistor is connected into the feedback loop between the output of the second amplifier stage and the input thereof. As an alternative, the resistor is connected between the output terminal of the analog amplifier, which is connected to the terminal pad, and the output terminal of the second amplifier stage.
In accordance with another feature of the invention, the second output-side amplifier section has an output and the resistor is connected between the output terminal and the output of the second amplifier section.
In accordance with a further feature of the invention, the second output-side amplifier section has an output and the resistor is connected between the output of the second output-side amplifier section and the node.
In accordance with an added feature of the invention, the resistor is connected between the output of the second output-side amplifier section and the node.
In accordance with an additional feature of the invention, the first input-side amplifier section is a differential amplifier.
In accordance with yet another feature of the invention, the analog amplifier is an operational amplifier or an operational transconductance amplifier.
In a development of the invention, a third amplifier stage is provided, which is connected between the output of the input-side differential amplifier and the input of the output-side amplifier stage. In such a case, a further feedback is connected between the output of the analog amplifier and the input of the third amplifier stage or the output of the input-side differential amplifier. Such a feedback path is embodied in accordance with the feedback path associated with the second amplifier stage.
In accordance with yet a further feature of the invention, there are provided a third amplifier section having an input and a signal path connected between the first input-side amplifier section and the second output-side amplifier section, a second series circuit having a second resistor, a second capacitive element, and a second node between the second resistor and the second capacitive element, the second series circuit connected between the output terminal and the input of the third amplifier section, and a further capacitive element connecting the second node to the terminal for the reference-ground potential.
In accordance with yet an added feature of the invention, the second output-side amplifier section is a differential second output-side amplifier section having an input, a third series circuit has a third resistor, a third capacitive element, a third node between the third resistor and the third capacitive element, the third series circuit couples the input of the differential second output-side amplifier section to the output terminal, and another capacitive element connects the third node to the terminal for the reference-ground potential.
In accordance with yet an additional feature of the invention, the second output-side amplifier section is a differential second output-side amplifier section having an input, a second series circuit has a second resistor, a second capacitive element, a second node between the second resistor and the second capacitive element, and the second series circuit couples the input of the differential second output-side amplifier section to the output terminal, and a further capacitive element connects the second node to the terminal for the reference-ground potential.
The capacitive element connecting the feedback path to reference-ground potential is, on one hand, to be given sufficiently small dimensions so that the transfer response of the feedback analog amplifier is not significantly impaired, i.e., the phase margin of its transfer function is not shifted toward lower frequencies. On the other hand, however, the capacitive element must be chosen to be large enough so that the interference radiations coupled in at high frequency are adequately attenuated. In practice, the time constant formed from the resistor and the capacitive element should be five to ten times greater than the transition frequency of the analog amplifier to achieve the advantageous effects mentioned above.
In the application of amplifiers, it is usually endeavored to keep the capacitive loading of the amplifier input as small as possible to achieve the highest possible transition frequency and sufficient gain even at high signal frequencies. In the case of the invention, by contrast, an additional loading is applied to the input of the output-side amplifier section of the analog amplifier by the capacitive element connected to reference-ground potential. Given suitable dimensioning, however, an improved behavior with respect to the coupling-in of electromagnetic interference radiation results, surprisingly, for such an amplifier disposed at the output of the integrated circuit.
With the objects of the invention in view, in an integrated circuit having an input terminal for feeding in an input signal to be amplified, an output terminal for outputting an amplified output signal, and a terminal pad connected to the output terminal for outputting the output signal, there is also provided an analog amplifier including a first input-side amplifier section having a first signal path and an output, a second output-side amplifier section having a second signal path, the first and second signal paths connected in series and a transistor having a control terminal coupled to the output of the first input-side amplifier section and a controlled current path coupled to the terminal for the reference-ground potential and to the output terminal, a series circuit having a resistor, a capacitive element, a node between the resistor and the capacitive element, and ends, one of the ends of the series circuit connected to the output terminal, and another of the ends of the series circuit connected to the control terminal and a further capacitive element connected between the node and the terminal for the reference-ground potential.
With the objects of the invention in view, there is also provided an analog amplifier for amplifying an input signal, including a terminal for a reference-ground potential, a first input-side amplifier section having a first signal path and an output, a second output-side amplifier section having a second signal path, the first and second signal paths connected in series and a transistor having a control terminal coupled to the output of the first input-side amplifier section and a controlled current path coupled to the terminal for the reference-ground potential, the controlled current path to be coupled to an output terminal for outputting an amplified output signal, a series circuit having a resistor, a capacitive element, a node between the resistor and the capacitive element, and ends, one of the ends of the series circuit to be connected to the output terminal, and another of the ends of the series circuit connected to the control terminal, and a further capacitive element connected between the node and the terminal for the reference-ground potential.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an integrated circuit with an analog amplifier, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.